This invention relates to the drilling of a well into the earth and more particularly is concerned with a technique for controlling lost circulation.
In the drilling of a well into the earth by rotary drilling techniques, a drill bit is attached to a drill string and the drill bit is rotated in contact with the earth to cut and break the earth and form a well therein. Drilling fluid is circulated between the surface of the earth and the bottom of the well to remove drill cuttings therefrom, lubricate the bit and drill string, and apply hydrostatic pressure in the well to control the flow of fluids into the well from earth formations penetrated by the well. Drilling fluids commonly employed include gaseous and liquid drilling fluids. Liquid drilling fluids, often called "drilling muds", may be, for example, water base, oil base, or emulsions. Drilling fluids are normally treated by including additives therein to provide desired rheological properties which make them particularly suitable for use in drilling wells.
In the drilling of a well into the earth, a problem sometimes occurs which is referred to as "lost circulation" and which involves the loss of unacceptably large amounts of whole drilling fluid into a formation penetrated by a well. Such a formation or portion thereof that accepts the drilling fluid is commonly termed a "lost circulation zone". Lost circulation may occur when the well encounters a formation of unusually high permeability or one which has naturally occurring fractures or fissures. Lost circulation may also occur by the inadvertent fracturing of a formation during drilling operations. Such fracturing sometimes occurs when the density of the mud is increased to provide a sufficiently high hydrostatic pressure to control high formation pressures. Other inadvertent fracturing of formations sometimes occurs because of fluctuations of the hydrostatic pressure imposed on formations due to movements of the drill string and drill casing in a well.
Many techniques have been used to control lost circulation. One such technique involves increasing the viscosity of a slug of the drilling fluid in order to increase its resistance to flow into the formation. Other techniques include the addition of bulk material, such as cottonseed hulls, sawdust, and ground walnut shells, to the drilling fluid to seal the lost circulation zone. Still other techniques involve the forming of plugs in a well to alleviate the loss of drilling fluid into the lost circulation zones.
In U.S. Pat. No. 2,801,077 to Howard et al. which reissued as U.S. Pat. No. Re. 24,942, there is described a technique of regaining circulation wherein a gel is formed in the well and forced into the lost circulation zone. This technique involves forming a slurry of a nonaqueous liquid and hydraulic cement and a clay and pumping the slurry down the well through a drill pipe while simultaneously pumping an aqueous liquid down the well so as to mix at the location of the lost circulation zone the slurry with the aqueous liquid. The clay and cement hydrate and form a gel which is forced into the lost circulation zone.
In U.S. Pat. No. 3,467,208 to Kelly, Jr., there is described a method of controlling lost circulation wherein an aqueous suspension of an oleophilic colloid which is both water and oil dispersible is circulated down a drill string in a well and into the wellbore where it is contacted with an oleaginous liquid to form a gel which tends to plug the formation into which lost circulation occurs.
In U.S. Pat. No. 2,800,964 to Garrick, there is described a process wherein a bridging material, a nonaqueous slurry of finely divided clay, and an aqueous liquid such as a drilling mud are combined within a well to produce a mixture in the well which is too viscous or plastic to be pumped but which can be displaced into a lost circulation zone. In U.S. Pat. No. 2,990,016 to Goins, Jr. et al., a concentrated slurry of a hydratable material in an inert carrier liquid is pumped down a borehole in one well channel and substantially simultaneously an aqueous liquid is pumped down the borehole through a second well channel. The two liquids are mixed in a borehole adjacent to the formation causing lost circulation to partially hydrate the hydratable material. A stiff sealing composition is formed by the partial hydration of the hydratable material and is squeezed into the lost circulation formation wherein further hydration of the hydratable material occurs, causing it to swell and harden to close the fissures.
In U.S. Pat. No. 3,724,564 to Messenger, there is described a method of controlling lost circulation by circulating down a well and into the vicinity of a lost circulation zone an aqueous mixture of a dispersing agent, an inert particulate material, and a water dispersible oleophilic colloid and mixing the aqueous mixture with an oleaginous liquid in the vicinity of the lost circulation zone to form a gel which tends to plug the formation into which lost circulation is occurring.
In U.S. Pat. No. 3,876,006 to Messenger, a method is described of alleviating lost circulation problems by mixing within the well and immediately above an incompetent formation an aqueous liquid and a slurry of a nonaqueous liquid and hydratable material to form an aqueous liquid-rich soft plug. This plug is displaced down the well and into the lost circulation zone to alleviate the loss of drilling fluid thereinto. In U.S. Pat. No. 3,987,855 to Messenger, there is described a method of controlling an active zone that is penetrated in the drilling of a well by forming a mixture of inert particulate material and an oil wetting dispersing agent in a light oil and circulating the mixture to a lower portion of the well and maintaining it there to allow the inert particulate material to separate from the light oil and form a plug in the well.